In this popup, fill in your API URL and Personal Access Token. After that, you will be able to attach any merge request to any Trello card on your selected Trello board.

What is my API URL?

Your API URL should be your GitLab instance URL with /api/v4 appended in the end of the URL.
For example, if your GitLab instance URL is https://gitlab.com, your API URL would be https://gitlab.com/api/v4.
If your instance's URL is https://example.com, your API URL will be https://example.com/api/v4.